In a known method of concentrated solar power (CSP) generation, radiation from the sun is focused, by such means as parabolic mirrors or heliostats, onto one or more solar radiation receivers that may be mounted, for example, on top of a tower, a so-called “solar tower” arrangement. The or each solar receiver absorbs the solar radiation as thermal energy and a working fluid with a high heat capacity, such as a molten salt, is used to transfer heat from the solar receiver to a heat exchanger in order to generate a further working fluid suitable for driving a prime mover. Typically, the prime mover is a steam turbine operating on the Rankine Cycle, though turbine fluids other than steam can also be used. Normally, the turbine or other prime mover drives an electrical generator for feeding power to a utility grid.
To transfer heat from the solar receiver to the heat exchanger, the working fluid is passed through channels that are in intimate thermal communication with the radiation absorbing elements of the or each receiver, and the fluid is then circulated to the heat exchanger through heavily insulated ducts to avoid excessive heat loss therefrom.
The efficiency of the conversion of solar radiation into useable power is of paramount importance. High efficiency is needed to allow CSP to compete, in terms of cost per unit of energy, against other forms of power generation, such as fossil fuel fired power stations. One aspect of the above CSP systems that has a pronounced effect upon the overall system efficiency is that of the temperature and pressure to which the turbine fluid is raised at the heat exchanger prior to being passed to the turbine. Thermodynamically, it is desirable to heat the fluid to as high a temperature and pressure as possible, so as to maximize the temperature and pressure difference across the turbine. The temperature and pressure achievable for the turbine fluid is, however, limited by the characteristics of the working fluid used to transfer thermal energy from the solar receiver to the heat exchanger. An example of a typical working fluid for CSP systems is a molten salt: a combination of 60% sodium nitrate and 40% potassium nitrate. This combination of sodium and potassium nitrate has a maximum working temperature of about 565° C., which temperature is not sufficient to generate super-critical steam at the heat exchanger for use as the turbine fluid. The useable temperature limits of molten salt mixtures are caused by crystallization at a lowermost temperature threshold and by salt decomposition at an uppermost temperature threshold. Although salts are known with decomposition temperatures that allow the temperature at the heat exchanger to rise beyond 565° C., salts having higher decomposition temperatures also have increased crystallization temperatures. An increase in the crystallization temperature results in an increased range of temperature for which the CSP system will not function because the working fluid will not flow around the system and is therefore not able to transfer energy from the solar receiver to the heat exchanger.
There is, therefore, a need for a means of transferring energy from a solar receiver, to a heat exchanger, over a wider range of temperature than is currently achievable using known arrangements of CSP systems.